Measurement by titration to the phenolphthalein end-point (pH 8.3) of that portion of
alkalinity arbitrarily attributed to all the hydroxyl content and half the carbonate content of a
titrand.
Notes: - For the carbonate system, with no other oxoanions, \(A_{\rm{P}} = [\ce{CO3^{2−}}] + [\ce{OH−}]\) where the concentrations refer to the solution before the titration \(\ce{CO3^{2−} + H+ -> HCO3−}\) and \(\ce{OH− + H+ -> H2O}\) performed to discoloration of the visual indicator.
- By definition, composite alkalinity is zero for solutions which have a pH value of 8.3 or less.
Source:
PAC, 2025, 97, 137. 'Glossary of terms for mass and volume in analytical chemistry (IUPAC Recommendations 2024)' on page 2 (https://doi.org/10.1515/pac-2023-0903)